The transaction fee collected from the block generation shall never be zero. Whatever technology we will have in the future, by the second law of thermodynamics, it is impossible to find a low probability event (here in bitcoin, to find a special hash) without consuming energy. As technology advances, mining rigs are approaching the theoretical energy consumption limit. These block generators will always need some Joule for their operation. Also, should the transaction fee be zero, these block generators would have no way to pay the energy bill after all 21M BTC are mined.
After all the 21M BTC are mined, the operation of these block generators is financed by selling the BTC they collect from the transaction fee. Suppose:
Then the ultimate BTC/Joule shall be C/F and ultimate BTC/USD exchange rate shall be C/F/B which is also EF/21000000/D .